1. Field of the Invention
This invention is concerned with an improved process for applying asphalt emulsion to an aggregate.
2. Description of the Prior Art
It has been the practice in road construction to select the type of asphalt emulsion to suit the aggregate used. Most aggregates used are predominantly electropositive or electronegative in the presence of water. When using an electropositive aggregate such as dolomite, an anionic (-) asphalt emulsion is employed because the negatively charged asphalt particles will be attracted to the electropositive aggregate and tend to be more readily deposited. On the other hand, a cationic (+) asphalt emulsion would be repelled by the aggregate and will not deposit on the aggregate until the water phase has evaporated.
When using an electronegative aggregate, such as the commonly used sandstones, the practice is to use a cationic (+) asphalt emulsion. An anionic (-) emulsion will not deposit on the aggregate until the water phase has evaporated.
By including a minor amount of a properly selected coupling agent, in accordance with this invention, in the water to wet down the aggregate prior to applying the asphalt emulsion, the application can be relatively independent of the type of aggregate. It is generally accepted that the electrochemical property of the aggregate determines whether good adhesion will be obtained with asphalt emulsions. This property is the nature of the electrical surface charge that the aggregate possesses when it is in contact with water. If the aggregate is dry, the electrical charges are practically nonexistent because adsorbed ions neutralize the electrical charge on the aggregate surface. In the presence of water, these ions dissociate from the aggregate surface, become dissolved in the water and the aggregate bears an electrical charge. When the aggregate and the asphalt emulsion are antagonistic, e.g., (+), good adhesion is attained if the coupling agent has a charge opposite the asphalt emulsion. In general, even when the aggregate and the asphalt emulsion are compatible, e.g., (-) vs. (+), adhesion can be enhanced when the coupling agent has a charge the same as that of the aggregate and opposite to that of the asphalt emulsion. It should be noted that the aggregate surface electrical charges range from those that are extremely electropositive to those that are extremely electronegative. Seldom does an aggregate have a surface charge that is entirely electropositive or electronegative because of the complex minerology involved. In most aggregates, both types of charges are present; for the aggregates may contain both silicon which will permit the surface to have some negative charges, and also calcium, iron, magnesium and/or aluminum which will allow the surface to have positive charges. Thus, the use of coupling agents will improve the deposition of asphalt onto typical aggregate with mixed electrical charges, such as basalts, porphyries, siliceous limestones, etc.